Communication can be regarded as a problem of recovering a desired signal from an input signal that includes undesired signals in addition to the desired signal. Undesired signals include random noise, as well as interference signals which are not random. In many cases, interference signals are generated within a communication system by hardware limitations. For example, when a 2-wire channel is used for full-duplex communication, the signal transmitted to the channel generates an interference signal (often referred to as “echo”) which interferes with reception of signals from the channel. The effect of interference signals which are generated within a communication system (e.g., echo) on reception can generally be reduced because such interference is derived from signals which are known within the communication system.
More specifically, one can regard a communication system as providing a parasitic system having an interference data stream as its input and the undesired interference signal as its output. For example, an imperfect hybrid in a 2-wire communication system provides such a parasitic system, where the transmitted data stream on the 2-wire channel is the interference data stream. Since communication systems are usually linear, the effect of the interference signal provided by the parasitic system can be reduced by deriving an appropriate correction signal from the interference data stream and adding the resulting correction signal to the received signal.
The correction signal is usually derived from the interference data stream by passing it through a linear filter. In cases where the parasitic system leading to the interference is time-invariant, the correction filter can also be time-invariant. Otherwise, the correction filter is usually time-dependent and is placed within a control loop for varying the filter parameters to minimize the contribution of the interference signal to the corrected received signal. Such filters are often referred to as adaptive filters.
Correction filters as described above are frequently implemented by passing the input signal though a tapped delay line. Each tap of the delay line corresponds to a different time delay applied to the input signal. For example, tap 1 could correspond to a delay of T0, tap 2 to a delay of 2T0, etc. In this architecture, the overall filter output is obtained by multiplying each tap output by a corresponding tap weight, and adding the resulting terms. Such filters are also known as transversal filters.
Transversal filters as described above have been known for some time, and thus numerous implementations are known in the art. For example, an extensive body of work relates to reducing the computation time required for digital transversal filters, which is mainly determined by the required multiplications. Such work includes the use of filters having tap weights that are exact powers of 2, so that multiplication can be performed by simple bit shift operations. In addition to such mathematical investigations, various physical implementations of transversal filters have been demonstrated. For example, in the common case where a transversal filter is implemented electrically, mathematical signals can be related to circuit voltages or to circuit currents.
Although voltage-mode transversal filters are more common than current-mode transversal filters, current mode filters can be advantageous in certain cases. U.S. Pat. No. 6,469,988 considers an example of a current-mode transversal filter used for echo cancellation in a communication system having binary modulation. However, many common communication systems employ non-binary modulation, and U.S. Pat. No. 6,469,988 does not consider such cases.
Another example of a current-mode transversal filter for echo cancellation is given in Lee et al., “A 125 MHz Mixed Signal Echo Canceller for Gigabit Ethernet on Copper Wire”, IEEE Journal of Solid State Circuits 36(3), pp 366-373, 2001. In this example, 5 level pulse amplitude modulation (PAM) is employed, and a single digital to analog converter (DAC) is used to provide multiplication at each tap. However, applicability to other modulation formats is not considered by Lee et al. Furthermore, in some cases, it is not practical to perform tap multiplication with a single DAC.
Accordingly, it is an object of the invention to provide current mode circuitry for interference reduction that is applicable to a variety of non-binary modulation formats. Another object of the invention is to provide current mode circuitry for interference reduction that can be used with various multiplier approaches.